1. Field of the Invention
The present invention relates to an imaging device and a focus control method thereof. In particular, the present invention relates to an imaging device and a focus control method thereof that carry out focus control at the time of imaging an imaged subject.
2. Description of the Related Art
In recent years, accompanying the increasing of the resolutions of solid-state imaging elements such as CCD (Charge Coupled Device) area sensors, CMOS (Complementary Metal Oxide Semiconductor) image sensors and the like, the demand for information devices having an imaging function, such as digital electronic still cameras, digital video cameras, cell phones, PDAs (Personal Digital Assistants, portable information terminals) and the like, has increased rapidly. Note that information devices having an imaging function such as described above are called imaging devices.
By the way, focus control methods that detect the distance to a main imaged subject include the contrast method and the phase difference AF (Auto Focus, automatic focus) method. The phase difference AF method may carry out detection of the focus position at high speed and with high accuracy, as compared with the contrast method, and therefore, is often employed in various imaging devices.
Note that it is known that the method of reading out in an imaging device that uses a CMOS is the rolling shutter method that carries out successive resetting and successive reading out from the top. In the rolling shutter method, because there is a time difference in the reading out timing in accordance with the pixel position, distortion arises in the image of the imaged subject in cases of an imaged subject that moves.
Accordingly, when imaging an imaged subject that moves by an imaging device that uses a CMOS, when it is made to carry out focus control by the phase difference AF method, the focus control is affected by the distortion due to the rolling shutter, and errors in phase difference detection arise due to image movement or image variations that arise during the lag in the read out timings.
Japanese Patent Application Laid-Open No. 2009-128579 discloses a device that, when reliable focal point detection results are not obtained at focal point detection pixels that are disposed in the horizontal direction, carries out focal point detection at focal point detection pixels that are disposed in the vertical direction, and, when movement of the imaged subject is detected, does not carry out focal point detection at focal point detection pixels that are disposed in the vertical direction.
Further, Japanese Patent Application Laid-Open No. 2008-72470 and Japanese Patent Application Laid-Open No. 2008-263352 disclose devices that controls such that charge accumulating timings of pixels for phase difference detection become the same.
However, in the technique disclosed in aforementioned Application Laid-Open No. 2009-128579, merely focal point detection by the phase difference AF method is carried out only under limited conditions, such as in accordance with the reliability of the focal point detection results or in a case in which movement of the imaged subject is not detected or the like, and, when focal point detection is carried out by the phase difference AF method, effects due to the rolling shutter cannot be reduced. Further, in the techniques disclosed in Japanese Patent Application Laid-Open No. 2008-72470 and Japanese Patent Application Laid-Open No. 2008-263352, costs are high because additional circuits are required.
The present invention provides an imaging device and a focus control method thereof that, even when carrying out detection of the focus position from signals read out from phase difference detection pixels that are disposed on different lines by the rolling shutter method, may reduce effects of distortion due to the rolling shutter while detecting the focus position, and may carry out focus control highly accurately, without providing additional circuits.